In most mammalian species the gluconeogenic enzyme, PEP carboxykinase, is found in both the cytoplasmic and the mitochondrial matrix intracellular compartments. The cytoplasmic form of the enzyme is known to be regulated by various hormonal and other physiological stimuli but little is known about the regulation of the mitochondrial isozyme. The present work is aimed at elucidating the possible regulatory factors for the mitochondrial form of the enzyme. Guinea pigs will be used for these studies and the relative rates of synthesis and degradation of the two isozymes will be compared at different physiological states using immunological and isotopic methods. The structural, immunological and mechanistic relationships of the two forms of the enzymes will also be examined. For this purpose, both forms of the enzyme have been already isolated from monkey and guinea pig liver. Finally, the mechanism of transport of the extramitochondrially synthesized mitochondrial form of the enzyme into its matrix location will be studied. Chicken liver, which contains only the mitochondrial isozyme will be used in these studies.